Field of the Invention
This invention relates to a method of controlling a respiratory assistance device and to a respiratory assistance device.
Description of the Related Art
Respiratory assistance devices are often used where a patient or other user requires assistance when breathing. This may be to assist in providing sufficient air to the patient during normal breathing or to treat a particular condition, such as sleep apnoea, for example. These may be used when in hospital, or at home, for example.
Depending on the treatment or patient needs, gases other than air may be used and/or used to supplement air. For example, air may be supplemented with oxygen to provide a patient with oxygen-enriched air. Other chemicals, compositions, or medicaments may also be added or alternatively used. Further, the gases may be humidified to provide for improved patient comfort.
Respiratory assistance devices typically comprise a respiratory gases source, such as a supply of pressurised gases and/or a gases flow generator such as a blower or compressor. The respiratory gases source delivers gases to a patient via an inspiratory conduit connected to a patient interface such as a full face mask, a nasal mask, or a nasal cannula, for example. The gases may be delivered at a pressure greater than atmospheric pressure to assist in delivering sufficient gases to the lungs of the patient and/or to provide respiratory support, for example, during treatment of chronic obstructive pulmonary disease (COPD).
During normal respiration (unassisted breathing), the pressure and flow rate within the airway of a patient vary. During inspiration, the pressure rapidly decreases from ambient pressure to a maximum negative pressure as the diaphragm moves downward. This causes air to be drawn into the airway with an increasing flow rate, initiating at near zero flow. As inspiration ends, the pressure returns toward ambient pressure and the flow rate returns to near zero. During subsequent expiration, the pressure rises as the diaphragm moves upward, generating increasing flow in the opposite direction (i.e., out of the airway). As expiration continues, the pressure tails off toward ambient pressure and the flow rate returns to near zero for commencement of the next inhalation.
Due to the physiological stress that a patient may experience in an intensive care unit (ICU), the patient can have a higher than usual demand for oxygen and often require a much greater level of ventilation compared to a normal, healthy patient. The patient can exert an increased level of effort of breathing trying to meet this demand and to ensure adequate ventilation. This increased level of effort means that the patient is expending greater metabolic effort and so producing more carbon dioxide (CO2). To remove the CO2 and keep respiratory gases concentrations at a safe level requires even greater ventilation, and therefore even greater effort. As the patient continually tries to increase the effort to breathe and is unable to adequately expire the increasing amounts of CO2 produced, the patient can experience respiratory muscle fatigue and develop hypercapnia.
Elevated levels of CO2 can also occur in a patient due to an obstructive disease such as asthma. Excess mucous in the airway and bronchoconstrictions can inhibit breathing and result in the respiratory muscles being unable to provide sufficient ventilation to meet the metabolic demands of the patient.
Respiratory assistance devices have been proposed which attempt to control their operation to better align with the changing needs of a patient during the breathing cycle.
For example, U.S. Pat. No. 5,148,802 describes an apparatus for providing alternating high and low positive pressures in the airway of a patient. The pressure levels are coordinated with the spontaneous respiration of the patient such that the disclosed apparatus provides lower positive pressure during exhalation than during inhalation, to decrease resistance experienced during exhalation. To achieve this, it is necessary for the disclosed apparatus to monitor the breathing cycle of the patient to determine when to switch between the pressure levels. Flow is monitored to determine instantaneous and average flow rates, with inhalation detected if the instantaneous flow rate is greater than the average flow rate and exhalation detected if the instantaneous flow rate is less than the average flow rate, and with the pressure levels being adjusted accordingly. While the disclosed apparatus may provide for some improved comfort for a patient, the instant inventors have recognised that the control provided thereby is not ideal. For example, during inhalation, when there is less resistance to the gases being delivered to the lungs since there is no opposing air flow as there is during exhalation, there may be no need to increase pressure.
The inventors have further recognised that flow resistance in the airway of a patient is proportional to the flow restriction in the nasal cavities of the patient. Indeed around 50% of the resistance to flow is provided by the nasal cavities. This nasal resistance is present during normal respiration and when the patient is provided with respiratory assistance. For example, during continuous positive airway pressure (CPAP) treatment, gases are delivered to a patient at an elevated pressure, but the resistance to flow remains the same. Consequently, the profile of pressure against gases flow rate during CPAP is the same as the profile during normal, unassisted respiration, but with the profile shifted on the pressure axis by the amount of pressure elevation. Prior art devices have at least not fully accounted for this relatively significant nasal resistance to flow, with flow being monitored based on measurements taken along the delivery path to, but prior to reaching, the patient.
It can be desirable to use a nasal high flow or jet delivery system, rather than a face mask based arrangement for some therapies. Further, some patients may prefer such interfaces based on perceived comfort levels. At present, such devices generally deliver gases at a fixed flow rate.
Object of the Invention
It is an object of the invention to provide a method of controlling a respiratory assistance device and/or a respiratory assistance device which overcomes or at least ameliorates one or more of the disadvantages of the prior art
Alternatively, it is an object to at least provide the public or industry with a useful choice.
Further objects of the invention will become apparent from the following description.